The correct answer is: A. isoquants where marginal product of one of the factors is zero.
An isoquant is a curve that shows all the combinations of two inputs that will produce the same level of output. The marginal product of a factor is the additional output that is produced when one more unit of that factor is used.
The ridge line of an isoquant is the locus of points on the isoquant where the marginal product of one of the factors is zero. This is because the marginal product of a factor is zero when the additional output produced by using one more unit of that factor is equal to the additional cost of using that unit.
If the marginal product of one of the factors is zero, then the firm will not use any more of that factor. This is because the firm can produce the same level of output by using less of that factor and more of the other factor.
The ridge line of an isoquant is important because it shows the maximum output that can be produced with a given amount of inputs. The firm will not produce any output on the ridge line because it can produce the same level of output with less inputs.
The other options are incorrect because they do not describe the ridge line of an isoquant. Option B is incorrect because the marginal product of two factors is never zero. Option C is incorrect because the marginal product of two factors can never be infinity. Option D is incorrect because the marginal product of two factors is not always equal to one.